This invention relates to the fabrication of a structure having a reliable, long-lived soldered electrical connection to a metallized ceramic substrate and, more particularly, to the fabrication of a structure with a flip chip electrically and structurally connected to the metallized ceramic substrate.
In one common architecture, a microelectronic chip is fabricated with large numbers of interconnected microelectronic circuits thereon. The microelectronic chip has input and output terminals of the microelectronic circuits on an external surface of the microelectronic chip. One form of the terminals is solder bumps which protrude above the external surface of the microelectronic chip.
A number of the microelectronic chips and other electronic devices may be supported on a ceramic substrate with metallized interconnections on the surface of the substrate. The metallization on the substrate provides interconnection between the microelectronic chips and also serves as a structural support for the fragile microelectronic chips. The substrate is typically made of a single-layer of ceramic material with the electrically conductive metallization traces on its surface, or multiple layers of ceramic material with electrically conductive traces on the various levels of the ceramic structure and vertical interconnects between the levels.
The microelectronic chip is attached to the substrate by affixing the solder bumps to the appropriate locations on the conductive traces of the substrate. The affixing is accomplished by fluxing the solder bumps and the traces, contacting the solder bumps to the metallization traces in an oven which heats the solder to a temperature above its liquidus temperature to cause it to reflow, and then cooling the assembly to below the solidus temperature allowing proper solidification of the solder, forming a conductive interconnect. The flux is thereafter removed. The solder joint between the solder bumps and the metallization on the substrate in this xe2x80x9cflip chipxe2x80x9d design serves both as the structural attachment and the electrical attachment of the microelectronic chip to the substrate.
The removal of the flux is a time-consuming task that adds to the product cost. The inventors have also recognized that the use of the flux may have adverse effects on the microelectronic chip itself. For example, where the microelectronic chip is based on silicon (Si) or gallium arsenide (GaAs) technology, the flux may contaminate the sensitive microcircuits. The flux residue may also result in environmental contamination. The inventors have recognized a need for a joining approach that overcomes these problems that arise from the use of flux-based solder joining of microelectronic chips to substrates. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for forming a soldered connection, and a metallized ceramic substrate that may be used in the method. The approach is particularly useful for forming solder joint electrical and structural connections between flip chips and metallized ceramic substrates, but it may be used more widely. The soldering method achieves the solder joining without the use of a flux and without a substantial modification to the composition of the solder.
In accordance with the invention, a method of forming a soldered electrical connection comprises the steps of providing a ceramic substrate having a thick-film metallization thereon, and depositing a joint-stabilizing, thin-film metallization overlying the thick-film metallization. The thin-film metallization comprises an adhesion layer overlying and in contact with the thick-film metallization, a readily wettable base-metal layer overlying and in contact with the adhesion layer, and an oxidation-prevention layer overlying and in contact with the base-metal layer. The method further includes providing an electrical conductor, and soldering the electrical conductor to the thin-film metallization of the ceramic substrate.
In a preferred application the electrical conductor is a bonding pad of a flip chip having a solder bump thereon. The step of soldering includes the steps of placing the bonding pad having the solder bump thereon in contact with the thin-film metallization, and heating the solder bump to a temperature greater than its liquidus temperature.
The adhesion layer is preferably a metal selected from the group consisting of titanium, titanium-tungsten, and chromium, and preferably has a thickness of from about 25 nanometers to about 100 nanometers. The base-metal layer is a metal selected from the group consisting of copper, nickel, and alloys thereof, and preferably has a thickness of from about 0.15 micrometers to about 5 micrometers, most preferably from about 0.75 micrometers to about 3 micrometers. The oxidation-prevention layer is a metal selected from the group consisting of gold and silver, and has a preferred thickness of from about 70 nanometers to about 600 nanometers, most preferably from about 100 nanometers to about 200 nanometers. The thick-film metallization comprises a precious metal selected from the group consisting of gold, silver, platinum, palladium, and combinations thereof.
The adhesion layer of the thin-film metallization ensures the adhesion of the overlying base-metal layer to the underlying thick-film metallization. Consequently, it is made relatively thin. The base-metal layer serves three functions. It is the layer to which the soldering is accomplished, it acts as an electrical conductor in the final assembly, and it also prevents diffusion of the underlying thick-film metallization into the liquid solder during the soldering step and into the solid solder during post-soldering processes and subsequent mission operations. Such diffusion would otherwise tend to convert the solder to a low-melting solid and reduce its creep-fatigue resistance. Maintaining good creep-fatigue resistance is an important consideration where the solder serves the structural function of attaching the flip chip to the metallized ceramic, and where the solder must retain its structural integrity through thermal strain cycles during post-soldering assembly processes and mission operation. The oxidation-prevention layer prevents oxidation of the structure during storage. Such oxidation, if it were to occur, would prevent soldering or would require fluxing of the metallization prior to soldering, which in turn would require cleanup of the flux after soldering is complete. The oxidation-prevention layer prevents such oxidation and allows subsequent good wetting during soldering without the use of a flux. The oxidation-prevention layer dissolves into the liquid solder during the soldering operation, and it is therefore made as thin as possible consistent with the need to achieve durable full coverage of the surface of the base-metal layer. Each of the layers of the thin-film metallization thus performs an important function in the thin-film metallization in cooperation with the other layers.
The present invention achieves high reliability of the soldered structure through a high resistance to chemical and mechanical, and therefore electrical, degradation of solder bump joints during subsequent assembly and in service operational environments.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.